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It depends on what the domain is all about.
If you are targeting a long-tail keyword, then it is not too long.
Why have a short domain which does not best describe what you are offering on the website?

Secondly are they dictionary words?
If you are using a combination of dictionary words, they a 3-words domain should be the most desirable.
3 words domains can be of any length.

No, it is not too long if it is a good domain name. Most of your visitors will be arriving at your website how? By manually typing in each letter of the domain name? No. Your visitors will click on a link from a search engine, a forum or other place, or bookmark your site in which case modern browsers will produce the URL after a few letters are typed in the address bar. So length really is not all that big of deal as almost nobody will be typing the domain name. I never type forum.digitalpoint.com. I just type "di" and my browser produces the URL from the bookmark and I click on it.

A URL shortener with a long domain name might not be so good of an idea. Other than that, the niche really doesn't matter too much.

And yes, I would remember WouldYouRememberThis.com. But if it was a .org or .net, I probably would get it wrong recalling it much later. Unless I have it bookmarked, in which case I would type a few letters and the browser would produce the URL for me to click on.

One thing missing here it seems is that this is more of a click world then when the net started and it was mostly offline pushing on business cards, flyers, TV commercials, where the URL was displayed.

Now, with the ads (banner and text), people posting recommendation on social media, any email lists, it is all click, so the old method of thinking that's been hanging around is essentially useless.
Some sites will still need to be short, sweet, and memorable, but not all that many any more.

As billzo points out, once it's bookmarked it's a snap to go back to with just the first few letters, so a 256 letter domain would work. History works that way too.

Also, the content and domain do not have to match even, as people aren't paying attention to things like that very often. It's more marketers paying attention. If people did pay that much attention do you think that they'd click links for some of the CPA nets that have something like zxujfs.com as the domain for things like acai berry products?

If you are selling to marketers the domain and product(s) should match, but otherwise I think that you are giving the people a bit more credit than they deserve on being concerned over such things.

In my opinion, it maybe too long enough, especially if it contains non memorable words. However, long or short it doesn't really matter too much. What really matter is the purpose of the domain.
Here are my few opinions about domain name:

For branding purpose, I prefer short and memorable domain.

For competition and niche targeting, I don't mind long domain name. However, the domain has to be easy to pronounce and quite memorable. I prefer long letters domain but easy to remember than short but hard to pronounce domain. But it just me, other may have different opinion.

The extension itself really matter. Usually I don't think too much on domain extensions (especially new extensions) but there are few extensions which i really like (due to easy to rank and targeted visitors).

So, in my opinion, twenty letters domain probably too long but it is still acceptable as long as the domain has memorable words on it. I have bough twenty letters domain last month and I am happy to buy it. You could read it here: Twenty four letters domain name (4 memorable words).